Surface Dislocation Research Articles

Synthesis, characterization, and gas sensing application of crumpled CdO sheets prepared in alkaline media

CdO crumpled sheets have been synthesized by microwave-assisted irradiation in alkaline solutions of NH4OH and NaOH, respectively, and in presence of a surfactant. The morphology of CdO samples have been investigated using Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). X-ray diffraction analysis (XRD) has been used to determine the cadmium oxy/hydroxide crystalline phases of CdO formed during synthesis and successive annealing at 300 °C. Crystallite sizes of CdO nanostructures obtained from NH4OH and NaOH reagent-assisted synthesis are 44 nm and 57 nm, respectively. The specific surface area (SSA), lattice parameter, X-ray density, dislocation density, macrostrain, and texture coefficient (TC), have also been determined. The optical characteristics of CdO samples were investigated using UV reflectance (DRS) and K-M plot. NO2 gas detecting capabilities of CdO nanostructures have been investigated at low working temperature of 150 °C. When compared to its NaOH counterpart, the CdO sample prepared by NH4OH reagent-assisted synthesis demonstrated a superior sensitivity to NO2, being demonstrated even at the low concentration of 0.14 ppm, with excellent selectivity.

Assessment of surface/subsurface damage in early-stage fatigue: A new attempt based on LCR wave

To facilitate a surface/subsurface fatigue damage awareness in the early stage, a signal-steady, thickness-independent, nondestructive method is proposed based on a critically refracted longitudinal (LCR) wave, which is compared with the conventional pulse-echo ultrasonic technique. With the evolution of cyclic damage in microscale, a normalized amplitude difference of LCR wave,Adif, increases monotonically, but the attenuation coefficient of the pulse-echo method evolves non-monotonically. The effects of surface roughness on the propagation of LCR wave are studied. A discrete strategy is proposed to detach the coupling between surface roughness and dislocation damping, and the mechanism is discussed with dislocation density-based calculation.

A Material View on Extrinsic Magnetic Domain Wall Pinning in Cylindrical CoNi Nanowires

Speed and reliability of magnetic domain wall (DW) motion are key parameters that must be controlled to realize the full potential of DW-based magnetic devices for logic and memory applications. A major hindrance to this is extrinsic DW pinning at specific sites related to shape and material defects, which may be present even if the sample synthesis is well controlled. Understanding the origin of DW pinning and reducing it are especially desirable in electrochemically deposited cylindrical magnetic nanowires (NWs), for which measurements of the fascinating physics predicted by theoretical computation have been inhibited by significant pinning. We experimentally investigate DW pinning in CoxNi100–x NWs by applying quasi-static magnetic fields. Wire compositions were varied with x = 20, 30, and 40, while the microstructure was changed by annealing or by varying the pH of the electrolyte for deposition. We conclude that pinning due to grain boundaries is the dominant mechanism, decreasing inversely with both the spontaneous magnetization and grain size. Second-order effects include inhomogeneities in lattice strain and the residual magnetocrystalline anisotropy. Surface roughness, dislocations, and impurities are not expected to play a significant role in DW pinning in these wire samples.

Research on the Mechanism of the Passive Reinforcement of Structural Surface Shear Strength by Bolts under Structural Surface Dislocation

In order to study the local deformation of an anchor bolt and the improvement in the shear strength of a structural surface under the misalignment of an anchorage structure surface, FLAC3D software was used to simulate granite, sandstone, and coal specimens with anchorage angles of 90° to analyze the damage of the anchoring agent and the changes in the local axial and shear forces of the anchor bolts with the misalignment of the structural surface. The results show that the anchor bolt near the structural surface had significant local characteristics with the misalignment of the structural surface; that is, the length of the local deformation area of the bolt was approximately equal to the length of the damaged area of the anchoring agent, and the stress on the anchor bolt was in a coupled tensile–shear stress state when the bolt reached the yield state. For the fully grouted bolts, it was this significant local feature that made the shear strength of the structural surface increase rapidly under a small shear displacement so that the structural surface reached a stable state. The improvement in the shear strength of the anchoring structural surface was caused by the misalignment of the structural surface. This is referred to as the passive improvement of the shear strength of the anchoring structural surface, which is the mechanism of the bonding section anchor to control the shear displacement of the structural surface and realize the stability of the rock mass.

Data-driven investigation of microstructure and surface integrity in additively manufactured multi-principal-element alloys

The great variability of the additively manufactured quality is becoming a major challenge to limit its wide application. Here, we combine the high-throughput molecular dynamics simulations with the artificial neural network to search for the high surface quality and damage minimization of the selective laser melted (SLMed) FeCrNi multi-principal-element alloy (MPEA), based on the complex microstructure obtained from experiment characterization. By adjusting the laser scanning speed and laser power density, a large amount of data related to the surface roughness, dislocation characteristics, and residual stress in the SLMed MPEAs is obtained for optimizing the SLM process. Atomic simulation results show that there are various degrees of Cr segregation under different processing parameters, which leads to the uncertainty of residual stress distribution and dislocation characteristics. Thus, the surface roughness and subsurface damage is highly sensitive to the SLM parameters. According to the training results of the artificial neural network, the laser scanning speed plays a more key role in regulating the surface integrity and subsurface damage in the SLMed MPEAs. The overall prediction results exhibit that the relatively slow laser scanning speed effectively reduces the residual stress. Importantly, a complex relationship between the SLMed parameters and properties is built. The current work provides a predictive recommendation to optimize the quality and process design of the SLMed MPEAs with high surface integrity and low subsurface damage.

Improving electrical transport properties of AlGaN/GaN heterostructure using AlSiN passivation

AlGaN/GaN heterostructures have been widely used in modern high power and high frequency device applications due to their excellent material characteristics. However, the existence of surface damages, defects and dislocations may degrade the device features. Here, we report the effect of AlSiN surface passivation on electrical transport properties in AlGaN/GaN heterostructure. Compared with the unpassivated, AlN and AlBN passivated AlGaN/GaN, better transport properties have been observed in the AlSiN passivated samples. Both the density and mobility increase in the AlSiN passivated AlGaN/GaN, which may be attributed to the influence of the AlGaN/GaN interface barrier height and suppression of trapping effects. The results show that AlSiN dielectric films is a very promising candidate as surface passivation for AlGaN/GaN HEMTs.

Dislocation Network‐Boosted PtNi Nanocatalysts Welded on Nickel Foam for Efficient and Durable Hydrogen Evolution at Ultrahigh Current Densities

AbstractLarge‐scale application of alkaline water electrolysis for high‐rate hydrogen production is severely hindered by high electricity cost, mainly due to difficulties to acquire cost‐effective catalytic electrodes with both extremely low overpotential and long‐term durability at ultrahigh current densities (≥1 A cm−2). Here it is demonstrated that by adopting a synthetic method of laser direct writing in liquid nitrogen via a commercial laser welding machine, a remarkably efficient and durable electrode with large area and low platinum content is obtained, where PtNi nanocatalysts with dislocation network are firmly welded on a nickel foam (NF). The dense dislocation network not only improves intrinsic activity of a majority of surface‐active sites induced by coupled compressive‐tensile strains synergistically promoting both Volmer and Tafel steps of alkaline hydrogen evolution reaction (HER), but also well stabilizes surface dislocations for HER at ultrahigh current densities. Such a robust electrode achieves record‐low overpotentials of 5 and 63 mV at 10 and 1000 mA cm−2 in alkaline medium, respectively, exhibiting negligible activity decay after 300 h chronoamperometric test at 1 A cm−2. It displays a high Pt mass activity 16 times higher than 20 wt% Pt/C loaded on NF, surpassing most of the recently reported efficient Pt‐based catalysts.

Displacive-Diffusive plasticity in nanoporous gold nanowires under tensile creep

While creep deformation of nanoporous gold (NPG) was argued to be mainly dislocation mediated, we show using molecular dynamics simulations of tensile creep of NPG nanowires that diffusion plays a significant role. Lower creep stress regimes are influenced by dominant atomic diffusion with negligible dislocation accumulation, while the interplay between surface diffusion and dislocation slips governs the deformation at higher creep stresses. The deformation mechanisms manifest themselves in the calculated activation volumes and creep stress exponents, which are of the same orders of those calculated from indentation creep experiments despite the different length and time scales. The contribution from the dislocation events decreases with increase in the ligament diameter, which is expressed as change in the creep exponents at the higher creep stresses. We suggest that this displacive-diffusion plasticity mechanism is essential to capture creep deformation of NPG structures.

Relationship between surface residual stress and dislocation configuration after laser shock processing of TC4 titanium alloy

Through the combination of simulation and experiment, this paper explored the correlation between residual compressive stress and loading pressure in the range of 2800 MPa to 11,000 MPa. The experimental results showed that the surface residual compressive stress increased first and then decreased with the increase of loading pressure. When the surface residual compressive stress decreased, the surface nanocrystallization appeared. With the increase of loading pressure, the form of energy dissipation changed. When the loading pressure was greater than 2800 MPa, which is equal to the Hugoniot elastic limit, the residual compressive stress layer was formed on the surface of material. When the loading pressure was greater than 5600 MPa, which is slightly more than the double Hugoniot elastic limit, the residual compressive stress layer was formed and the dislocation configuration changed. When the loading pressure was greater than 8500 MPa, which is slightly more than the triple Hugoniot elastic limit, the residual compressive stress decreased and the surface nanocrystallization occurred, accompanied by surface damage. For the new grains and sub grains formed by surface nanocrystallization, the atomic arrangement direction in the grains was consistent with the propagation direction of laser shock wave.

From Electric Doping Control to Thermal Defect Nucleation in Perovskites

AbstractSimultaneous optical microscopy and electric transport measurements on La0.7Sr0.3MnO3 nanobridges, grown on SrTiO3 (STO), show direct evidence of directional oxygen vacancy migration under large voltage bias. Comparative study on discontinuous structures, with voltages applied across a micron‐scale gap, demonstrates that high electric fields induce electrolytic modifications confined to the anode. Extensive electromigration is shown to induce the formation of linear surface scars on the STO substrate, following well defined crystallographic directions. The reproducible triggering of these surface dislocations is demonstrated, unveiling their thermal rather than electrical origin and it is shown that they do not represent electroformed conduction channels. High‐resolution scanning transmission electron microscopy imaging reveals that the scars correspond to nanometer scale steps caused by (101) gliding planes in the STO caused by the proliferation of edge dislocations and local lattice expansion. These findings shed light on selective electrically‐controlled atom migration and its role in structural modification of functional oxides, opening a unique avenue for ionotronic device design.

Synthesis, structural, optical, thermal, electrical, nonlinear optical and theoretical investigation on 3-aminopyridine picrate (3APP) single crystal

A single crystal of 3-Aminopyridinium picrate (3APP), an organic nonlinear optical (NLO) crystal is successfully grown using a slow evaporation technique with a dimension of 18 × 4 × 3 mm3. Single crystal XRD measurement confirms that 3APP crystal belongs to the monoclinic crystal system with space group P21/n. Powder XRD analysis is performed to estimate the phase purities and crystalline perfection of the grown 3APP crystal. UV-Vis-DRS spectral analysis of the grown 3APP crystal shows 97% of transmittance and cut-off wavelength at 511 nm with an optical band gap (Eg) of 2.31 eV. The presence of various functional groups of 3APP crystal is identified qualitatively by FTIR study. The etching study is performed on the grown 3APP crystal in order to identify surface morphology and dislocation density. The elemental composition of the 3APP crystal is confirmed by using EDS analysis. Thermal studies indicate that the 3APP crystal is stable up to 198 °C. The high LDT of 10.56 against an Nd: YAG laser shows the suitability of the 3APP crystal for high-power laser device applications. Dielectric measurements are carried out to examine the dielectric properties and electronic polarizability values of the 3APP crystal. Third order NLO parameters of the 3APP crystal obtained by Z-scan techniques are n2 = 3.53 × 10−12 m2/W, β = 1.25 × 10−5 m/W and χ(3) 3.90 × 10−9 esu (5.46 × 10−17(m/V)2). Here β and n2 are positive, due to the two-photon absorption nature and self-focusing nature as detected in closed and open aperture z-scan spectra. Theoretical calculations are performed using density functional theory with B3LYP/6-311++G(d,p) level and related to experimental results. Furthermore, the Frontier FMO, MEP and NBO have been visualized and studied. The results suggested that the 3APP crystal is suitable for NLO, optoelectronic, optical switching and optical limiting applications.

GaN-Djoser Pyramidal Self Powered UV Photodetector for Optical Signal Detection in Rugged Environments

Wireless communication under harsh environment using ultraviolet radiation remains a vital field of research. We have reported the novel GaN pyramids of Djoser ultraviolet photodetector operable at high-temperature conditions. The GaN-pyramids of Djoser ultraviolet photodetector display extremely high performance, detect weak optical signals and display self-powered behavior even under challenging conditions. GaN-pyramids of Djoser was grown via modulating flux ratio (Ga/N) in molecular beam epitaxy. Two-terminal asymmetric Pt electrodes metal semiconductor metal-ultraviolet photodetector were fabricated for better thermal stability and low dark current. The device exhibits high responsivity of 11.1 × 103 mAW−1 at RT and 5.6 × 103 mAW−1 at 250 °C with a fast response time of ∼ 200 µs in self-power mode. In photoconductive mode, the device demonstrates very high responsivity ∼ 2.5 × 107 mAW−1, high detectivity (1012 Jones) at a weak optical signal, while the device shows very high responsivity ∼1 × 107 mAW−1 at 250 °C. The extraordinary performance of the device is attributed to its unique Djoser-pyramidal structure with decreased surface stress and dislocations. The self-powered device with significantly high responsivity and capability to detect weak optical signals demonstrates the feasibility of a highly efficient photoreceiver design of GaN- pyramids of Djoser for ultraviolet communication, which can work under harsh environmental conditions.

Effect of V–III Ratio‐Based Growth Mode on the Surface Morphology, Strain Relaxation, and Dislocation Density of AlN Films Grown by Metal‐Organic Chemical Vapor Deposition

Herein, regulation of the growth mode through manipulating the V–III ratio during metal‐organic chemical vapor deposition (MOCVD) growth of AlN films on c‐plane sapphire substrates and optimization of the crystalline quality and surface morphology of AlN films are demonstrated. Based on detailed analyses of the evolution process of surface morphology, dislocation annihilation, and stress state of the AlN film during MOCVD growth, it is found that 2D rather than 3D growth mode introduces tensile stress as well as promotes edge‐type threading dislocation (ETD) propagation and crack generation, while 3D growth mode produces rough surface. These results would help to further understand the AlN growth mechanism and provide important guidance on the preparation of AlN‐related epitaxial structure.

Study on surface nanocrystallization and dislocation configuration of E690 steel strengthened by laser shock peening

Through the combination of simulation and experiment, this paper explored the direct relationship between laser energy and surface nanocrystallization of E690 high strength steel, and analyzed the indirect relationship between surface residual compressive stress and surface nanocrystallization. The experimental and simulation results showed that when the laser energy reached 10J, grain refinement occurred on the surface of E690 high strength steel, and the maximum residual compressive stress appeared on the sub surface of the material. At the same time, with the increase of laser energy, violent vibration and extrusion occurred between lattices, resulting in a certain micro thermal effect and inducing the climbing of screw dislocations. Therefore, there was a qualitative relationship between the surface nanocrystallization of E690 high strength steel after laser shock processing and the distribution of residual stress, that was, when the maximum residual stress appeared on the sub surface, the surface of material appeared nanocrystallization. This was because surface nanocrystallization consumed a lot of energy, which reduced the surface residual stress. This study was of great significance to predict the dislocation configuration changes of E690 high strength steel after laser shock processing.

Effects of Cutting Force on Formation of Subsurface Damage During Nano-Cutting of Single-Crystal Tungsten

Abstract Single-crystal tungsten is widely utilized in various fields, benefiting from its outstanding properties. Nano-cutting, as an ultra-precision machining method, can realize high efficiency and low damage. However, from the atomic perspective, the formation mechanism of subsurface damage during the nano-cutting of tungsten is still unclear. Herein, the molecular dynamics (MD) simulation of nano-cutting single-crystal tungsten was established to elucidate the evolution of subsurface damage and the effects of cutting force on subsurface damage. The corresponding results showed the existence of damage including atomic cluster, vacancy defect, “V-shaped” dislocation, stair-rod dislocation, and dislocation ring on the subsurface during the cutting. There were dislocation lines in 1/2<111>, <100>, <110>, and other directions due to plastic deformation dominated by dislocation slip, and the 1/2<111> dislocation lines could be merged into stable <100> dislocation lines under certain circumstances during the cutting. The variation of cutting force and cutting force fluctuation induced by changing cutting parameters had a great influence on the subsurface damage of tungsten, including the number of surface defect atoms, dislocation density, and thickness of the subsurface damage layer. In nano-cutting of single-crystal tungsten, a smaller cutting depth and appropriate cutting speed should be selected to reduce subsurface damage. This study provides an insight into the evolution mechanism of subsurface damage of tungsten and is high of significance for achieving low-damage machining of tungsten components.

Environmental photochemistry with thiol- and silica-modified plasmonic nanocomposites: SERS sensing of municipal solid waste and tannery waste leachate from groundwater.

Mismanagement of obsolete solid waste generates a massive deteriorating effect on the environment. There is a high level of open trash disposal contaminating its neighboring water bodies. This despoliation trash causes an endangerment to the living environment. The waste management act is to hinder harmful effects on human beings, animals, plants, and their natural environment through the principles of waste prevention, waste processing, and waste disposal. Surface-enhanced Raman scattering (SERS) enhances the hazardous chemical sensing of environmental pollutants. To vigorously focus on the leaching of a couple of landfills in groundwater and surface water, an unusual combination of SERS-based poly vinyl thiol and silica-modified silver nanocomposites (PVT/SiO2@Ag NCs) was synthesized. The optical, crystalline, and structural properties of PVT/SiO2@Ag NCs were described with UV-visible spectroscopy (UV-Vis), X-ray diffractometer (XRD), transmission electron microscope (TEM), and energy-dispersive X-ray analysis (EDX). The surface plasmon resonance (SPR) is detected at 403nm from the PVT/SiO2@Ag NPs. The average crystallite size of PVT/SiO2 @ Ag NCs is estimated using the Scherrer formula as 11nm. The calculated specific surface area (SSA), strains, and dislocation densities demonstrate the improved mechanical properties of the substrate. The well-separated spherical shape of NPs is also observed, and the composition of silica and sulfur element in addition of Ag was confirmed by EDAX. Negatively charged SiO2 were bound strongly with the SH group and Ag NPs through electrostatic interaction mechanism as S-Ag-O-Si-O-Ag-S. SERS sensitivity is demonstrated by the prepared nanoparticles using an environmentally ignored leachate of municipal solid waste (MSW) and tannery waste (TW) landfill. PVT/SiO2@Ag NCs has detected the presence of innards of MSW leachate viz., aromatic hydrocarbon, phenols, phthalates, and pesticide from the groundwater. Furthermore, the TW leachate compositions of benzenes, hydrocarbons, amines, and chromium VI were analytically identified. Also, the leaching of TW leachate was confirmed in the water samples referred. Hence, this study provides a novel SERS sensor of PVT/SiO2@Ag NCs in the tile to detect and analyze environmentally ignored organic and inorganic compounds.

Atomistic modeling of surface and grain boundary dislocation nucleation in FCC metals

Dislocation nucleation plays a critical role in the plastic deformation of crystalline materials. However, it is challenging to predict the active mode and associated rate of dislocation nucleation under typical experimental loading conditions through molecular dynamics simulation due to timescale limitations. Here we use the free-end nudged elastic band method to determine the activation energies and activation volumes of dislocation nucleation in four typical face-centered cubic metals of Au, Al, Cu and Ni. We focus on the representative processes of surface and grain boundary dislocation nucleation. The atomistically determined activation volumes of these dislocation nucleation processes are larger than 10b3 (with b being the Burgers vector length) under typical experimental loading conditions. These results are compared with experimentally measured activation volumes in ultrafine-grained and nanocrystalline metals, thereby providing mechanistic insight into their rate-controlling deformation mechanisms.

Strain and Defect Evolution of Si1-xGex/Si Heterostructures Grown by Pulsed Laser Induced Epitaxy

The relaxation mechanism of Si1-xGex/Si heterostructures subjected to pulsed laser melting was investigated by probing the pulsed laser induced epitaxy (PLIE) regime of undoped 20 nm Si0.5Ge0.5/Si thin films. The pseudomorphic “critical thickness” and evolution of bi-layer formation was determined as a function of average Ge concentration of the films via quantitative analysis of (004) HRXRD rocking curves. Comparison of pseudomorphic thicknesses alongside SIMS analysis reveals a “dynamic critical Ge concentration” of 27-30% Ge as the PLIE limit for pseudomorphic growth that is independent of average Ge concentration of the films. Plan-view weak-beam dark-field imaging revealed that surface dislocation half-loops are the primary strain relieving defects that reach concentrations on the order of 1010 cm−2. It is theorized that quasi-cellular solidification leads to lateral Ge segregation, creating nm scale localized regions of Ge pile-up and stress concentration. The morphology of the liquid/solid interface along with stress localization is what allows for the dislocation half-loop to be the primary strain relieving defect, with <110> edge defects acting as secondary. These results are important for understanding the conditions and strategies necessary to utilize pulsed laser melting to its fullest potential in applications towards pMOS source/drain contact engineering.

Precision-improving manufacturing produces ordered ultra-fine grained surface layer of tungsten heavy alloy through ultrasonic elliptical vibration cutting

High-precision and ultra-fine grained surface of tungsten heavy alloy exhibits superior service performance that is useful for many applications and shows promises for use as key parts in nuclear protection and precision instruments. The present study concentrated on a kind of precision-improving ultrasonic elliptic vibration cutting approaches, which fabricated nanometer-level surface roughness, inhibited subsurface damages evolution, and formed continuous ultra-fine grained layer microstructure. The surface morphologies have been characterized by ultra-depth three dimensional microscope and white light interferometer. Excellent machined surface quality was achieved under ultrasonic elliptic vibration cutting condition, and an ideal surface roughness of Sa = 70.7 nm was obtained. Microstructural alteration studied using EBSD technique and TEM observation confirmed the generation of ultra-fine grained structure. Surface grain size has been reduced from 50 ∼ 100 μm to 50 ∼ 300 nm without cracks and other micro-damages. Research demonstrated that surface energy accumulation and dislocations clustering induced by high-strain rate diamond tool impact provided the primary driving force of ductile-mode removal and grain recrystallization. A dislocation density-based simulation model was carried out to complement the static experimental investigations. The present work on surface formation and microstructural evolution identified that ultrasonic elliptical vibration machining has potential to deliver improved tungsten-based alloys service performance.

Mechanism of low Ohmic contact resistance to p-type GaN by suppressed edge dislocations

In this paper, an excellent Ohmic contact to p-GaN with a low specific contact resistance (ρc) of 2.0 × 10−5 Ω·cm2 is demonstrated using a patterned sapphire substrate (PSS) and oxidized Ni/Au contacts. GaN epitaxy with high crystal quality on the PSS, confirmed by high-resolution x-ray diffraction, played a key role in the improved Ohmic contact to p-GaN. The edge dislocations were annihilated during the epitaxial process on the PSS to afford a low surface dislocation density, which was in accordance with the results of transmission electron microscopy and cathodoluminescence spectroscopy. Furthermore, a reduced Fermi level and enhanced activation efficiency of Mg with suppressed segregation around the dislocations were demonstrated by Kelvin probe force microscopy and contact Hall measurements, respectively. A GaN p-channel metal oxide semiconductor device fabricated on the PSS displayed a twofold higher forward current density and superior gate controllability compared with that fabricated on a conventional sapphire substrate.